The present invention relates to the production of ammonia, and the corresponding synthesis gas, from a hydrocarbon containing feedstock. The industrial synthesis of ammonia is achieved through a synthesis gas containing a molal ratio of H.sub.2 /N.sub.2 as close as possible to the stoichiometric value, that is 3.0, and some quantities of inert gases such as methane and argo, which one tries to reduce to the minimum. Processes for the production of ammonia by steam reforming of hydrocarbons are described in U.S. Pat. Nos. 2,829,113, 3,278,452, 3,264,066, 3,388,074 and 3,442,613.
In the process most commonly used for the production of ammonia synthesis gas, starting from a feedstock ranging from natural gas to naphtha, the total feedstock undergoes first a primary steam reforming reaction, at a temperature of about 800.degree. C. and under a pressure of between about 25 to 35 bars, inside refractory tubes containing a reforming catalyst and externally heated by a set of burners. In said primary steam reforming, the amount of steam used in the reaction is usually expressed by the steam/carbon ratio, which is the number of moles H.sub.2 O per atom of hydrocarbon carbon; said ratio is very often higher than 3.0, and usually closer to 4.0. The gas produced by the primary reforming reaction is then subjected to a secondary reforming reaction, in the presence of a reforming catalyst, at about 1000.degree. C. and under the same pressure, in a reactor operating under essentially adiabatic conditions, by reacting with the amount of air which is just necessary to obtain in the final synthesis gas a molal ratio of H.sub.2 /N.sub.2 equal 3.0. Said temperature of about 1000.degree. C. is imposed by the need to have less than about 0.6 percent methane in the gas effluent from said secondary reforming, on a dry gas basis, in order to avoid excessive purge rates in the ammonia synthesis loop. Consequently, as the amount of air introduced in said secondary reforming is limited by the stoichiometry of the ammonia synthesis, it is found that the effluent gas temperature from the primary steam reforming should be above a certain minimum, which is about 800.degree. C. The effluent gas from the secondary reformer is then treated for shift conversion of CO to CO.sub.2, then scrubbed with an appropriate solvent to remove essentially all the CO.sub.2 it contains, then methanated to convert into methane essentially all the residual carbon oxides it contains, which are poisons to the ammonia synthesis catalyst.
The above described conventional process for the production of ammonia synthesis gas has basically two major drawbacks. Firstly, a large amount of steam must be used in the primary steam reforming reaction, that is a steam/carbon ratio of at least about 3.0, due to the minimum ratio allowable to prevent carbon formation on the reforming catalyst, and in order to obtain an acceptable methane content at the outlet of the primary and secondary reformers. Said large amount of steam penalizes the conventional process in two ways, because not only does it represents an energy burden, but also because of the high investment required in the equipment to produce said steam. Secondly, because of the minimum temperature of about 800.degree. C. at the outlet of the primary reformer, the metallurgy of the tubes used in said reformer requires restricting the operating pressure of the process to about 40 bars; consequently, a large amount of energy is required to compress the final synthesis gas produced, the volume of which is about four times that of the feedstock, to reach the ammonia synthesis pressure, usually comprised between 180 and 380 bars.
The process described in U.S. Pat. No. 3,4442,613 has the merit of avoiding the aforementioned second drawback, by using in the secondary reforming an amount of air appreciably larger than that required by the stoichiometry of the ammonia synthesis, the excess nitrogen being removed downstream in a cryogenic separation step. However, the amount of steam to be used in the primary reforming of this process is as large as that of the conventional process. Furthermore, the process of U.S. Pat. No. 3,442,613 operates with a methane content in the feed to the secondary reformer of less than 50 percent by volume, on a dry gas basis, which means the process does not take full advantage of reducing the degree of reforming in the primary reformer. Furthermore, said process requires the expansion of the process gas in a gas expander to produce the refrigeration required in the cryogenic purifier, which is an appreciable loss of energy, especially when considering that the expansion of hydrogen produces much less refrigeration than that of other gases such as nitrogen.
The process described in U.S. Pat. No. 3,278,452 permits the direct production of an ammonia synthesis gas with the stoichiometric composition, while achieving a substantial saving on the steam required for the primary reforming, because only a fraction of the feedstock may be treated in the latter, the other fraction going directly to the secondary reformer. Nevertheless, this process requires the use of oxygen enriched air in the secondary reformer, and the production of said oxygen is expensive in investment and in energy consumption. In addition, said process requires two or several steps in the secondary reforming reaction, with oxygen injection at the inlet of each step. Now it is virtually impossible to build such a system, because the oxygen to be injected in the second and subsequent steps is mixed with a gas at very high temperature coming from the first step, and this is a source of very elaborate technological problems; not only very special and expensive materials of construction must be used in the mixing zone, but also expensive provision must be made in the design for access to said zone for maintenance purposes. Furthermore, as the temperature of the reacting gas mixture increases regularly from the first to the last catalyst bed, the total volume of catalyst thus required is appreciably larger than that needed to carry the same reaction in a single step: in the latter case, all the catalyst would be at very high temperature, higher than or equal to the outlet temperature, because the reaction would be initiated before reaching the catalyst bed, thus raising appreciably the temperature of the mixture, and it is well known that the higher temperature of the catalyst increases appreciably the reaction rate and therefore reduces the volume of catalyst required.
The main object of the present invention is precisely to avoid simultaneously the two above mentioned drawbacks of the conventional process, that is on one hand to reduce appreciably the operating temperature of the primary reformer, which consequently allows an increase in the operating pressure and to reduce the corresponding fuel requirements, and on the other hand to reduce appreciably the amount of steam required for the process, thereby achieving an overall energy saving.
Another object of the present invention is to achieve a saving in investment as compared to the conventional steam reforming process for the production of ammonia.
Still another object of the present invention is to replace part of the expensive fuel required in the conventional process, that is a fuel with low sulfur and heavy metals contents, by electric power which would be cheaper if produced in power plants using coal or nuclear energy.
Furthermore, in the conventional process, the purge gas from the ammonia synthesis loop is either used as fuel or treated in costly additional facilities to recover the hydrogen therein. Another object of the present invention is to upgrade the hydrogen contained in said purge gas without the need to invest in additional facilities.